1. Field of the Invention
The present invention relates to a processing system and more particularly relates to a processing system, such as a semiconductor manufacturing system, that includes a scheduling system for scheduling processing of lots based on a composite ratio which focuses on selected process scheduling factors such as, processing system efficiency, such as, providing continuous work to constraint resources in the processing system, and on other process scheduling factors such as customer factors and market factors which may influence scheduling of processes.
2. Description of the Related Art
Processing systems such as manufacturing systems often utilize one or more manufacturing lines or process paths, each having a variety of equipment stations to convert lots of material into useful articles. The lots of material may be any raw material, processed material, components, or other tangible item. Typically, each lot is transported to stations, and each piece of equipment at each station carries out a specific operation or process on the lots. In a reentrant processing system, one or more process paths proceed from a station and reenter or return to that station at some later time for further processing subsequent to processing at that station or at another station. For example, in a semiconductor manufacturing system, lots, which in such case are generally silicon based wafers, may flow along a process path of a cleaning equipment station, photo lithographer equipment station, to an etching equipment station, to an implanter equipment station, return to the cleaning equipment station, return to the photo lithographer equipment station, and so on until a complete integrated circuit is manufactured.
Careful scheduling of process operations at stations in process paths is generally essential to such a system operating efficiently. Processing systems such as semiconductor manufacturing facilities and other facilities where process paths include multiple equipment resources are generally especially sensitive to lot processing scheduling issues. Thus, industries have expended a tremendous amount of effort in extensively utilizing resource planning to increase processing efficiency by appropriately scheduling processing of lots.
Some sophisticated scheduling systems coordinate facility wide planning by globally defining the manufacturing process. Such facility wide planning typically includes identifying constraint resources, also commonly referred to as bottleneck operations. An equipment station is considered a constraint resource if its remaining available production capacity is relatively small such as less than about five per cent (5%). It is a generally accepted theory that manufacturing efficiency and thus overall manufacturing system capacity is determined by constraint resources. The need to avoid starvation, i.e. idle time or queue depletion, of a constraint resource, is discussed in Goldratt, Eliahu, "Theory of Constraints". Thus, an overall approach to increasing the efficiency of a manufacturing system attempts to ensure that constraint resources are "fed", i.e. lots are provided to the constraint resource, by ensuring that queues of lots are replenished at the constraint resource so as to keep usage of the constraint resource as close to one hundred per cent (100%) as possible.
In the manufacturing process, queues of lots are distributed among various equipment resource stations in accordance with the process stage of each particular lot. The process scheduling of such lots are often driven solely by customer due dates. However, dispatching of lots at the stations for processing by the respective stations solely in accordance with customer due dates does not always result in optimal constraint resource utilization especially in a highly reentrant processing system. Additionally, conventional scheduling systems especially in highly reentrant processing environments, have typically proved inadequate in maintaining high usage of constraint resources.